1. Technical Field
The present invention relates to a torque detecting device for detecting rotational torque applied to a rotary shaft, and more particularly relates to a torque detecting device that can be suitably used to detect steering torque in an electric power steering apparatus.
2. Description of Related Art
In an electric power steering apparatus which assists steering by driving a steering assist motor according to the rotation operation of a steering member, such as a steering wheel, and applying rotational force of the motor to a steering mechanism, it is necessary to detect the steering torque applied to the steering member for use in the drive control of the steering assist motor. For this detection, in the prior art, the electric power steering apparatus uses a torque detecting device which is mounted in the middle of a steering shaft connecting the steering member and the steering mechanism together.
In this torque detecting device, the steering shaft (rotary shaft), which is an object of detection, is divided into a first shaft located on the steering member side and a second shaft located on the steering mechanism side, the first and second shafts are connected coaxially with a small-diameter torsion bar as a torsion spring, and the steering torque (rotational torque) applied to the steering shaft by the rotation operation of the steering member is detected on the basis of a relative angular displacement caused between the first and second shafts with the torsion of the torsion bar.
For the means for detecting the relative angular displacement between the first and second shafts, various kinds of structures have been conventionally proposed. As one example, there is a torque detecting device comprising a cylindrical magnet which rotates together with one of the first and second shafts; a yoke ring which rotates together with the other; and detecting means which uses a change in a magnetic circuit formed between the cylindrical magnet and the yoke ring (see, for example, Japanese Patent Application Laid-Open No. 2004-125717).
In the yoke ring, a plurality of pole claws extending in the axial direction are arranged at equal intervals in the circumferential direction on one side of a ring-shaped yoke body, and a pair of ring-shaped yoke bodies with their pole claws being arranged alternately in the circumferential direction are fixed to the first shaft or the second shaft. The cylindrical magnet is a multi-polar magnet including the same number of pairs of magnetic poles as the pole claws of the yoke rings arranged in the circumferential direction, and positioned and fixed to the second shaft or the first shaft so that, when the cylindrical magnet is in a neutral state in which there is no relative angular displacement between the first and second shafts, the pole claws of the yoke rings are aligned on the boundaries between the N and S poles.
On the outside of the two yoke rings, magnetic flux collecting rings for collecting the magnetic flux generated in these yoke rings are placed closely to face the yoke bodies, respectively. These magnetic flux collecting rings have magnetic flux collecting sections extending in the axial direction from the magnetic flux collecting rings, at positions aligned in the circumferential direction, and a magnetic sensor composed of a magnetic detection element such as a Hall element is placed between these magnetic flux collecting sections opposing each other with a predetermined air gap therebetween. With a mold member formed in a cylindrical shape to cover the outside of the magnetic flux collecting rings, the magnetic flux collecting rings and magnetic sensor described above are integrated while maintaining their positional relationship, and fixed to a housing supporting the steering shaft through the mold member.
In this structure, when steering torque is applied to the steering shaft and a relative angular displacement is caused between the first and second shafts, the positional relationship in the circumferential direction between the pole claws of the two yoke rings and the magnetic poles of the cylindrical magnet changes in mutually opposite direction, and the leakage flux in the air gap between the magnetic flux collecting sections of the magnetic flux collecting rings is increased or decreased by the change of magnetic flux in the respective yoke rings corresponding to the positional change. By extracting a change in the output of the magnetic sensor corresponding to this increase or decrease, it is possible to detect the steering torque.
In the torque detecting device constructed as described above, when magnetic noise applied from outside are superimposed on the magnetic flux collected in the magnetic flux collecting rings, the output of the magnetic sensor suffers from the influence of the noise, and there is a possibility that the accuracy of detecting the steering torque is lowered. Thus, in the prior art, a magnetic shield for shielding magnetic noise is provided in the periphery of the magnetic flux collecting rings.
FIG. 1 is a cross sectional view showing the structure of a conventional magnetic shield disclosed in Japanese Patent Application Laid-Open No. 2004-125717. FIG. 1 shows a magnetic flux collecting ring assembly comprising a pair of magnetic flux collecting rings 6 and 6 and a magnetic sensor 7 which are integrated with a mold member 61 formed by molding a resin into a cylindrical shape. The magnetic flux collecting rings 6 and 6 are integrated so that they are separated from each other by a predetermined distance in the axial direction and exposed to the inner circumferential surface of the mold member 61. The magnetic flux collecting rings 6 and 6 have, at corresponding positions in the circumferential direction, magnetic flux collecting sections 60 and 60 extending toward each other in the axial direction, and the magnetic sensor 7 is positioned between the opposing surfaces of the ends of the magnetic flux collecting sections 60 and 60 which are bent outward in the radial direction.
As shown in FIG. 1, the magnetic shield 9 is a thin plate of magnetic material composed of an outer circumferential plate 9a for covering the outer circumferential surface of the mold member 61 and linked end-face plates 9b and 9b, which are formed by bending both sides of the outer circumferential plate 9a inward at substantially right angles for covering the end faces, is mounted to cover three surfaces (the outer circumferential surface and both end faces) except the inner circumferential surface of the mold member 61 in which the magnetic flux collecting rings 6 and 6 made of magnetic material are exposed, and performs the function of shielding magnetic noise coming from various directions as shown by the arrows in FIG. 1 and eliminating the influence on the magnetic flux collecting rings 6 and 6.
As disclosed in Japanese Patent Application Laid-Open No. 2004-125717, such a magnetic shield 9 is mounted by a procedure in which a belt-like plate having a U-shaped cross section consisting of the outer circumferential plate 9a and end-face plates 9b and 9b shown in FIG. 1 is wound around the outside of the mold member 61, cut at an appropriate position, and joined to the beginning end of the belt-like plate. During this mounting, however, a lot of power and work are required to bent and wind the belt-like plate having the U-shaped cross section and further work is required for joining after winding, and thus there is a problem that the product cost is increased by an increase in the number of assembly steps.
It is also possible to construct the outer circumferential plate 9a and the end-face plates 9b and 9b as separate rings and mount them by a procedure in which the outer circumferential plate 9a and the end-face plates 9b and 9b are positioned separately on the outside of the mold member 61 and then joined together. In this case, however, it is necessary to join the outer circumferential plate 9a and the end-face plates 9b and 9b over the entire circumference, and thus there is no effect of reducing the number of assembly steps.